Only after an exceptional spring tide do their tops briefly emerge from the mud, and even then they remain exposed for roughly an hour before the estuary swallows them again. The structure extends up to 200 metres to the north-east of a pier and around 70 metres to the south-west, with what have been described as side shoots branching away from the main line at intervals. It is a fish weir, a fixed trap designed to intercept fish as the tide rises and falls, funnelling them into an enclosure from which they cannot easily escape.

The weir has not been definitively dated, but it sits within a broader pattern of estuarine fishing activity in this part of the Shannon system. Several fish weirs associated with the nearby islands have been traced to the medieval period, and some connected to Canon Island, which lies immediately to the south-east of Inishmacowney, have been dated to between the mid-thirteenth and early fifteenth centuries. The posts themselves are slender, roughly six centimetres in diameter and spaced about twenty centimetres apart, a construction method consistent with weirs designed to trap fish by allowing water through while holding the catch. Whether this particular structure belongs to the same medieval horizon as those on Canon Island, or represents something earlier or later, remains an open question. It was brought to the attention of the National Monuments Service by Fintan Ginnane.

The island is also known as Horse Island, and the weir runs along the edge of the mud flats that characterise this sheltered stretch of the estuary. Because the posts only surface briefly after exceptional spring tides, actually seeing the structure requires a good deal of patience and timing, and even then what appears is modest, a low broken line of worn timber rather than anything obviously dramatic. That modesty is rather the point. This is the working infrastructure of a medieval or post-medieval fishing economy, preserved not through any deliberate act of conservation but simply by the estuary mud that has kept it submerged and largely intact for centuries.